1. Field of the Invention
The present invention relates to a method of manufacturing an ND filter, the ND filter manufactured by the method, and an aperture device and a camera having the ND filter. In particular, the present invention relates to a method of manufacturing an ND filter suitable for a photographing optical system such as a video camera or a still video camera, the ND filter manufactured by the method, and an aperture device having the ND filter.
2. Related Background Art
An aperture device is arranged in an optical path of a photographing optical system in order to control the amount of light incident on a silver halide film or a solid-state image pickup element such as a CCD, and when an object field is bright, the aperture device is stopped down to have a smaller aperture size.
On a very fine day, or when a high-luminance object field is to be photographed, the aperture device is stopped down to have a small aperture size, and is easily influenced by hunting phenomenon of aperture and diffraction of light, resulting in deterioration of image quality. As a countermeasure against the deterioration, a film-like neutral density (ND) filter is attached to each aperture blade so as to increase the aperture size of the aperture device even if brightness of an object field is the same.
In recent years, the density of the ND filter is increased according to an increase in sensitivity of an image pickup element so as to further decrease a transmittance of light and increase an aperture size of the aperture device even if the brightness of the object field is the same. However, when the density of the ND filter is increased in this way, a difference is increased between the amount of light “a”, which is transmitted through an ND filter, and the amount of light “b”, which is not transmitted through the ND filter in a state shown in FIG. 37. Then, a “shading” phenomenon occurs, in which the brightness varies across a screen, and resolution is undesirably decreased. In order to eliminate this drawback, the ND filter must have a structure in which its density is changed so as to sequentially increase the transmittance toward a center of an optical axis.
Incidentally, FIG. 37 shows a photographing optical system having a conventional aperture device. In the figure, lenses 906A, 906B, 906C, and 906D constitute a photographing optical system 906. Reference numeral 907 denotes a solid-state image pickup element, and 908 denotes a low-pass filter. In addition, reference numerals 911 to 914 denote members constituting the aperture device; 911, an ND filter; and 912 and 913, aperture blades, which are moved to oppose to each other. Those two aperture blades are driven in a vertical direction on a paper surface of the figure and form a substantially rhombic aperture. The ND filter is adhered to the aperture blade 912. Reference numeral 914 denotes an aperture blade support plate.
In general, as a method of manufacturing an ND filter, there are a method of mixing and kneading a light absorbing organic dye or pigment in a film-like material (cellulose acetate, polyethylene terephthalate (PET), vinyl chloride, etc.), and a method of coating a light absorbing organic dye or pigment onto the material. With those manufacturing methods, a filter having a uniform density can be manufactured. However, it is very difficult to manufacture a gradation ND filter in which density changes depending upon the portion in the filter.
Concerning such a gradation ND filter (density variable ND filter), the inventors of the present invention have proposed a method of manufacturing a gradation type ND filter according to a microphotography in Japanese Patent Application Laid-Open No. 05-281593 (no corresponding US patent), U.S. Pat. No. 2,771,078, and Japanese Patent Application Laid-Open No. 06-175193 (no corresponding U.S. patent). In a video camera in those days, improvement in image quality was realized by an ND filter manufactured by this method. However, in order to cope with high sensitivity, miniaturization, high image quality of a CCD in recent years, image quality may be deteriorated due to an influence resulting from diffusion of light by silver halide particles, particularly in use under special conditions (e.g., a state of a small aperture size under backlight).
In addition, Japanese Patent Application Laid-Open No. 11-38206 (no corresponding U.S. patent) discloses a method of manufacturing an elliptical gradation ND filter using a vacuum evaporation method. However, with this method, there is a drawback in that a density cannot be changed in a very small area (e.g., change in transmittance from 3% to 80% in a range of 3 mm).
Moreover, as a measure for coping with the above-mentioned high image quality, ND filters with single density are adhered to a plurality of aperture blades and driven to generate parts where the ND filters overlap with each other and parts where the ND filters do not overlap with each other. Thus, it becomes possible to change a density. However, with this method, there are drawbacks in that, for example, cost is increased due to increase in the number of ND filters to be used, thickness of a device is increased due to existence of a plurality of ND filters in aperture blades to make it impossible to cope with tendency of miniaturization and space-saving.
Furthermore, in recent years, a stepwise multi-density ND filter has started to be used. However, it has been found that, when an optical path length changes in areas with different densities, a transmission phase changes by a degree equivalent to the change to cause deterioration of image quality (decrease in resolution). The deterioration of image quality is the largest when a transmission phase difference is λ/2, and is relatively small when the transmission phase difference is λ/4 to λ/8 (λ: 540 nm). In order to eliminate such a phenomenon, provision of a layer for correcting an optical path length is necessary.